The general rule of thumb I use is not to go above 90% of my MHR, which would be 171 for you. And really most of your work out should be in the more moderate range, interspersed with higher intensity making up a smaller portion of your total cardio workout for interval training. You might find this thread helpful Target Heart Rate !?
. If you are new to aerobic exercise or havenít done any in a long time it possible you cardio-vascular system is just a bit out of practice and you need to build it up gradually, be a little patient with yourself. It can be a wonderful motivator, seeing how exercise is benefiting your health not just your figure, especially on those days and weeks when the scale doesnít want to cooperate.